Tetraazamacrocyclic Cobalt Catalyst/Semiconductor Hybrid Systems And Photocathodes For Photo(Electro) Catalytic Hydrogen Production

Hydrogen production by solar-driven water splitting is an ideal approach to solve worldwide problems of rapid fossil fuel depletion and excessive carbon dioxide emission.To this end,it is very important to develop energy-efficient,cost-effective,stable,and environment-benign photocatalytic systems and photoelectrocatalytic(PEC)devices for H2 production from water.The modification of the surface of semiconductor with molecular redox catalysts is an effective way to boost the efficiencies of photocatalytic systems and PEC electrodes for H2 evolution reaction(HER).However,to date,very limited molecular catalysts have been immobilized to the surface of semiconductors for photocatalytic and PEC HER,and most of the reported hybrid photocathodes still suffer from low efficiency and poor stability.This thesis focuses on construction of more efficient and stable hybrid systems and devices by immobilizing molecular catalysts to the surface of narrow band-gap semiconductors by using a stable anchor for photo(electro)catalytic H2 production.The research was made around following aspects:(1)An anchoring group-functionalized tetraazamacrocyclic cobalt catalyst,[Co(CR-DCP)X2]+(C1,CR=2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentaene,DCP=2,6-dicarboxypyridyl,X=Br),was successfully synthesized,which was chemically immobilized to the surface of CuInS2/ZnS(CISZ)core-shell quantum dots(QDs)through a 2,6-dicarboxypyridyl(DCP)multidentate anchoring group to construct the C1@CISZ hybrid system.The photocatalytic H2 evolution performance of C1@CISZ was studied in pH 4.5 ascobic acid/sodium ascobate aqueous solutions under visible light illumination.The results showed that the H2 evolution rate of C1@CISZ was 3 orders of magnitude higher than that of bare CIS-Z and 3-fold higer than that of the simply mixed system of unfuncionalized macrocyclic cobalt(C2)and CISZ.Long-time photolysis experiments showed that the C1@CISZ hybrid system has better stability than the C2+CISZ mixed system.Moreover,the electron transfer kinetics and charge separation efficiency of these systems were comparatively studied by photoluminescence(PL)and transient absorption(TA)spectroscopy.The results demonstrated that surface-bound C1 evidently accelerated the interfacial electron transfer,and meanwhile retarded the charge recombination process on the surface of QDs.(2)The highly active and stable cobalt molecular catalyst C1 was immobilized to the surface of Si/TiO2 to construct a Si/TiO2/C1 hybrid photocathode.The performance of Si/TiO2/C1 for PEC H2 production was studied in pH 4.5 acetate buffer under visible light illumination.The results showed that compared to bare Si/TiO2,the onset potential of Si/TiO2/C1 was positively shifted by 0.32 V and the photocurrent density at 0 V vs RHE was raised by a factor of 31,reaching-0.68 mA cm-2,which is higher than the J(0 v)values reported for molecular catalyst-modified Ga/In-free hybrid photocathodes.In addition,Si/TiO2/C1 displayed a steady photocurrent density of-0.57 mA cm-2 during photoelectrolysis at 0 V vs RHE over 10 h.The stability of Si/TiO2/C1 is superior to the previously reported molecular catalyst-modified hybrid photocathodes.The charge transfer kinetics at the surface of Si/TiO2/C1 photoelectrode was studied by intensity modulation photocurrent spectroscopy(IMPS).The results revealed that the surface-bound cobalt catalyst could not only evidently accelerate the interfacial electron transfer rate,but also suppress the surface electron-hole recombination,which substantially improved PEC H2-evolution activity of the Si/TiO2/C1 hybrid photoelectrode.(3)To explore the influence of anchors on interfacial electron transfer,PEC H2-evolution activity and stability of hybrid photocathodes,tetraazamacrocyclic cobalt catalysts(C1,C3-C5)functionalized with different anching groups were synthesized.These cobalt catalysts were immobilized to the surface of Si/TiO2 through DCP,COOH,PO3H2,and Py anchors,respectively,and a series of hybrid photocathodes were constructed.IMPS studies demonstrated that the hybrid photoelectrode formed by coordinate attachment of the pyridyl N atom to coordinately unsaturated surface Tin+ cations exhibited faster interfacial electron transfer rate than the ones formed by covalent bonding of COOH and PO3H2 anchors with the surface Ti-OH units,and Si/TiO2/C1 with a DCP anchor,which can not only covalently bond to the surface Ti-OH sites but also coordinately attach to the surface Tin+ cations,displayed fastest forward electron transfer and slowest electron-hole recombination.As such,Si/TiO2/C1 displayed the highest activity and best stability for PEC H2 evolution under visible light illumination among the tested hybrid photocathodes with other anchoring groups.(4)To enhance the photo voltage of hybrid photocathodes,the cobalt catalyst C1 was immobilized to the surface of Cu2O/CuO heterojunction to construct a Cu2O/CuO/C1 hybrid photocathode.The performance of Cu2O/CuO/C1 for PEC H2 evolution was studied in pH 6.8 and 0.1 M Na2SO4 solution under simulated one sun illumination.The optimized Cu2O/CuO/Cl electrode displayed an onset potential more positive than 0.7 V vs RHE and a photocurrent density of-2.0 mA cm-2 at 0 V.IMPS results verified that the surface-bound cobalt catalyst improved the PEC performance of the Cu2O/CuO electrode by simultaneously facilitating interfacial electron transfer and retarding charge recombination at the solid/electrolyte interface.The long-time PEC experiments showed that Cu2O/CuO/C1 was not stable under test conditions due to the photocorrosion of Cu2O/CuO,which led to a low faradaic efficiency(24%).Therefore,the Cu2O/CuO-based hybrid photocathodes need to be further stabilized by coated with a protecting layer to effectively prevent it from photocorrosion.